(1) Field of the Invention
The invention relates to the general field of photolithography with particular reference to correction of image distortion.
(2) Description of the Prior Art
The proximity effect is a form of optical distortion associated with photoresist images. For a given development time, whether or not a given area of a photoresist layer will be left or removed after the development process depends on the total amount of energy deposited in that area during its exposure to radiation. Image features whose size and/or separation approach the resolution limit of said radiation will thus be subject to distortion that depends on how the diffraction maxima and minima, that lie on both sides of a `sharp` edge, interact with one another.
The proximity effect can be compensated for, at least in part, by modifying any given feature in the opposite direction to the expected distortion. Thus, a line that would otherwise come out too narrow can be drawn as wider than its true width, etc. The data that represents the information from which a mask suitable for use in photolithography can be generated, is stored in a data file so corrections to allow for the proximity effect will also be stored there. The overall nature and scope of these corrections, and how they get into the file, will vary with the application and the user.
The optical proximity correction (OPC) is commonly calculated by summing two Gaussian functions whose value depend on a critical dimension (CD) defined by the design rules as well as on the wave-length of the exposing radiation. In general, the distortion of lines that are part of a dense assemblage will be more positive than the distortion of isolated lines in optical mode. There are, broadly speaking, three approaches that can be taken: (1) the CD for dense lines may be increased while the isolated lines are left unchanged (2) the CD for isolated lines may be decreased while the dense lines are left unchanged or (3) the CD for the dense lines may be increased while that of the isolated lines is simultaneously decreased.
Application of these various approaches is further complicated by the fact that results will additionally depend on the particular process that is being used. For example, some depth of focus (DOF) of isolated lines would be lost if the i-line process, involving exposing radiation with a wavelength of 365 nanometers (nm), is used whereas DOF will be enhanced in the Deep Ultraviolet (DUV) process which involves an exposing radiation wavelength of 248 nm.
In FIG. 1, we show an idealized line pattern made up of two main sections--a densely populated area 1 and an area 2 which contains only isolated lines. Line crowding as exemplified by 1 is typical of what is found in cell areas of an Integrated Circuit (IC) design whereas lone isolated lines such as 2 are typically found in the peripheral areas of ICs.
FIG. 2 shows how the pattern of FIG. 1 appears when transferred to photoresist on a plane wafer. The isolated or peripheral lines 2 can be seen to be slightly wider as well as slightly shorter than they were in the original mask (FIG. 1). In this case the photoresist image was produced using the i-line process. The image of FIG. 1 was subjected to OPC prior to its transfer to photoresist but, as seen, some distortion nevertheless remains.
FIG. 3 shows another example of how the pattern of FIG. 1 appears when transferred to photoresist on a topography wafer. In this case, the isolated or peripheral lines 2 can be seen to be slightly narrower or slightly wider as well as slightly shorter than they were in the original mask (FIG. 1). In this case the photoresist image was produced using the DUV process. The image of FIG. 1 was also subjected to OPC prior to its transfer to photoresist.
Various approaches have been taken in the prior art to dealing with the proximity effect. Eisenberg et al. (U.S. Pat. No. 5,057,462 October 1991) examine the results of a first pass attempt and then modify the etching and resist parameters accordingly. Borodovsky et al. (U.S. Pat. No. 5,498,579 March 1996) use two masks, the second one serving to compensate for proximity effects introduced by the first one. Liebmann (U.S. Pat. No. 5,553,273 September 1996) sorts a design into small areas according to shape and width and those areas that are identified as gate regions are biased based on applicable OPC rules. Maehara (U.S. Pat. No. 5,375,157 December 1994) teaches the manufacture of a distortion free mask for X-ray lithography. Chung et al. (U.S. Pat. No. 5,432,714 July 1995) show how accumulated information on exposure can be used during electron beam lithography to compensate for proximity effects. As will become clear, none of these methods offer the simplicity of application provided by the present invention.